A cap of the type stated by way of introduction is known from patent specification GB-A-1,571,938. More specifically, this document discloses temporary, hermetic sealing between an opening end surface on the spout of a container and an oxygen-tight liner, which is mounted in the cap, and permanent joining of the liner and the cap, preferably by induction heating of plastic layers on both sides of the liner after the mounting of the cap on the container.
A first drawback of this method is that the mounting of the liner in the cap requires precise positioning of the cap before the insertion of the liner, i.e. it requires a special mounting unit, which first turns the cap right and then inserts the liner. A second drawback is that, no matter where in the production line this mounting unit is placed, there is still a risk that a liner will fall out of the cap again, thus making a hermetic seal between a container and the defective cap impossible to obtain.
A manufacturing method, other than that disclosed in GB-A-1,571,938, is described in DE-A1-196 21 617. According to this patent specification, which relates to a screw cap, a liner is inserted into a mould, in which the liner is positioned in a manner which is not described in greater detail. Then plastic is injected into the mould, thereby causing a special separation layer on one side of the liner to be melted with the plastic material of the cap. Finally, the cap is mounted on the spout of a container, where the liner is glued by means of a glue thread applied to the end surface of the spout. When removing the cap from the container, the liner remains on the container, since the adhesive force of the glue thread exceeds the adhesive force of the separation layer of the liner.
A first drawback of the solution according to DE-A1-196 21 617 is that, irrespective of the type of container, it is necessary to apply the glue thread in connection with the mounting of the cap on the container and that this application of the glue thread constitutes an extra operation, which has to be carried out before the mounting of the actual cap. A second drawback is that the liner, which remains after the removal of the cap, prevents quick opening of the container. Furthermore, the remaining cap makes it more difficult in particular for disabled people to open the container. In addition, it is stated in DE-A1-196 21 617 that the liner is inserted into the mould when making the cap, but it is not stated where in the mould it should be placed.
In these circumstances, it is reasonable to assume that the liner is supposed to be placed in the mould in the same way as in previously known patent specification U.S. Pat. No. 4,774,134. According to this document, a liner having a first flat side of a meltable plastic material is placed with an opposite second flat side on a core of a mould, after which the mould is closed and plastic is injected round the core and the liner placed thereon. This causes the plastic and the plastic material of the liner to melt together, thus being permanently joined.
A first drawback of the solution according to U.S. Pat. No. 4,774,134 is that an extra operation has been added after the manufacturing of the cap, in which operation an annular packing in the form of a string of sealing material is injected into the cap. A second drawback is that the positioning of the liner on the core of the mould slows down the cooling process of the core when the mould is open, which may result in a considerably longer cycle time. The vacuum channels for retaining the liner on the core also make it more difficult to provide coolant channels in the core, which channels could contribute to a reduced core temperature and thus a reduced cycle time.
Another cap in the form of a screw cap, which has some features in common with the cap stated by way of introduction, is already known from patent specification U.S. Pat. No. 5,927,532. More particularly, this specification discloses a child-resistant cap, which in one embodiment has an insert with a thin liner, which may be welded to an end surface of the spout on a container to obtain oxygen-tightness. When opening the container, the weld joint is broken by lifting the insert at the same time as the cap since the insert is pulled along by a circumferential bead in the cap. It is thus possible to remove the cap and to break the oxygen-tight seal in one single grip.
A first drawback of this prior-art cap is that it is comparatively difficult to manufacture, since it consists of several parts that have to be assembled in a separate mounting operation. A second drawback is that the lifting motion to break the seal has to be carried out by pulling the container and the cap apart, which requires a relatively great force and often results in a sudden opening motion, which may cause spillage of part of the contents of the container. Another drawback is that the division of the opening operation into two separate steps makes it difficult to know if the seal has been broken or not.